BUTE. GEOLOGY. 



that change occurring in the immediate vicinity of the 

 trap, the influence of which it bespeaks. Here it 

 acquires a mottled colour, presenting purple and red 

 stains from the intermingled shales ; which all present 

 the same appearance of high oxydation in their constituent 

 iron ; while at the same time it acquires an aspect so 

 argillaceous that it can scarcely be recognised as a 

 limestone. It is unfit for building, but the purest parts 

 are selected for agricultural purposes. 



The last of the rocks is the trap. This has been 

 shown, in the general sketch, to constitute nearly the 

 whole of the Garroch head, but there is also another 

 much inferior portion which will afterwards be described. 



The general disposition of the trap at the Garroch 

 head has already been mentioned; and, in examining 

 into the causes of these ridges and prolonged vallies, 

 it will be found that the whole promontory consists of 

 a series of beds of which the direction is north-westerly ; 

 while they dip to the south-west in an angle of about 

 fifteen degrees. They present a perfect appearance of 

 parallel stratification ; their abrupt edges declining from 

 the perpendicular in an angle equal to that of their 

 dip, and often forming high inland cliffs of greater or 

 less extent, prolonged on the line of their bearing. It 

 will be seen, on reverting to the position of the sand- 

 stone, that these beds lie over that rock in a conformable 

 manner ; and they possess an almost absolute parallelism 

 throughout. The overlying position is distinctly seen 

 on the west side of the island, less perfectly on the 

 east. Among the numerous instances of apparent stra- 

 tification in trap which have occurred in the course of 

 this survey, the present is infinitely the most regular 

 and perfect ; since, as far as I could perceive, it does 

 not in any instance betray indications of a disposition 

 different from that of the sandstone on which it lies. 

 Yet, as its contact with this rock is but imperfectly 

 visible, it must not be concluded that such irregularities 



VOL. II. II H 



